The RS6 will have a heavy engine. The twin turbo, bigger radiator, the intercooler, as well as extra reinforcements for the engine block to cope with the stress and strains from the extra energy generated from the combustion process will add weight to the car. I am sure the gearbox and drive shafts will need to be upgraded to handle the 1000Nm plus of torque. The Quattro system adds another 200kg to the cars weight i think.

Unless Audi offset this weight increase by using lightweight body panels and suspension parts, this RS6 will become a heavy beast.